This invention to feeding pulverulent materials into a conveying line. More particularly, this invention relates to a process for feeding a second stream of pulverulent materials into a conveying line carrying a first controllable flow of pulverulent materials. One application, among many, of the present invention is the feeding of dust removed from blast-furnace gas into a flow of pulverized coal.
In systems for cleaning blast-furnace gas, the solid pollutants are separated from the gaseous phase by means of dry separators such as, for example, dust catchers, cyclones, bag filters and electrostatic precipitators. These solid residues are collected in hoppers installed directly beneath the dry separators.
These hoppers, which must be emptied regularly, freely discharge the solid residues by means of discharge devices either directly into railway wagons or lorries, or simply onto a pile beneath the hoppers. The solid residues are then loaded by mechanical shovels into railway wagons or trucks and are transported to a landfill site. It should be noted that the solid residues removed from blast-furnace gas mainly consist of iron dust and coke.
The discharging of the solid residues from the separator hoppers is a very dusty operation which without doubt, causes problems from the point of view of health and safety at the workplace and raises considerable environmental protection issues. Additionally, the dumping of the solid residues in the open air releases, in an uncontrolled manner, harmful or toxic gases and vapors, which are conveyed out of the gas cleaning system by the solid residues when the hopper is emptied. These uncontrollably released gases and vapors without doubt, represent a considerable safety problem. It is clear that this discontinuous handling of the solid residues is an unhealthy, polluting and expensive practice. To avoid having to dispose of these solid residues in landfills, consideration has been given to feeding them back into the blast furnace. It would appear that an ideal means of feeding these solid residues is clearly the system for injecting pulverized coal into the blast furnace through the tuyeres of the blast main. It would appear in fact, that the system for injecting large quantities of pulverulent materials into the blast furnace would be the ideal vehicle to convey the above discussed solid residues. If this system could be used for re-injecting the dust into the blast furnace, an ideal means of recycling the materials contained in the dust would be available and the cost of landfilling this dust would be eliminated.
The simplest method would clearly be to mix the dust with the coal in the storage bins and to inject a mixture of dust and coal into the blast furnace. However, this solution has several disadvantages. The coal storage bins are normally located at some distance from the blast furnace and the gas cleaning equipment. It would therefore be necessary to convey the dust from the cleaning plant to the storage bins and then transport it back to the blast furnace. As the dust is much more abrasive than the particles of coal, this method would cause rapid wear of the coal conveying lines. Moreover, the quantity of coal injected could not be precisely monitored, as the coal concentration is neither known nor constant.
Another potential solution would be to inject the dust into the main pneumatic conveying line carrying the pulverized coal at a point close to the blast furnace. This solution would avoid the useless conveying of dust, and wear on the pipes, due to dust abrasion, would be minimized.
However, the injection of coal is an important parameter in the operation of a blast furnace. It is therefore essential to be able to monitor precisely the flow of coal injected at any moment and it is necessary to avoid disturbing the coal injection rate by introducing a second product stream into the pulverized coal flow.
It is therefore desirable to provide a process which would enable a second stream of pulverulent materials to be fed in a controlled manner into a line carrying a first controlled flow of pulverulent materials, without disturbing this first flow.